Generally, a spark plug tester has an electrical circuit which includes a high voltage capacitor connected with an output of a D-C high voltage generator through a high resistance, and said capacitor is further connected in parallel with a spark plug to be tested. Specifically, in such a circuit, the capacitor is charged through the high resistance so that the terminal voltage of the capacitor is gradually raised. When the terminal voltage reaches a sparking level, the spark plug fires upon electrical discharge of the capacitor whose potential is instaneously dropped to the low level, seasing the discharge. The terminal voltage of the capacitor thereafter sets out gradual rising by charging, and when it gets to the sparking level, an instantaneous discharge is generated by sparking. As the circuit of the tester repeats the foregoing sequence, terminal voltage of the capacitor varies in the form of saw-tooth pulsatile wave, which is a phenomenon called relaxation oscillation. In such an electrical variation, the voltage level at the time of completion of discharge by sparking, namely the lowest voltage level, is much smaller than the sparking voltage level of the upmost potential in the above-mentioned pulsatile wave, and becomes a substantially constant value irrespective of the sparking voltage level. In respect of such varying voltage characteristic, the testing for whether sparking voltage level of the plug is proper or not, may be performed by observing an average value of pulsatile voltage with a D-C indicator.
However, a spark plug tester utilizing the preceding principle has a fatal defect in misleading a wrong plug as good one. In case a current flow leaks in a tested plug due to adherence of soot and dust particles thereto or due to a high humidity, the maximum voltage in the capacitor is equivalent to a value determined by dividing output potential of the D-C high voltage generator into two portions through the aforesaid high resistance and leakage resistance of plug. If the capacitor voltage does not reach the sparking level of tested plug, relaxation oscillation is not generated and therefore testing fails to observe an average value of varying voltage. However, even in this case, the D-C indicator represents the divided voltage level of the capacitor, and if the divided level is equal to the proper average value of pulsatile voltage obtained in testing of a good plug, the wrong plug is mistaken as good one.